DESCRIPTION: The mandibular implant/mucosa supported removable overdenture has been shown to be an effective treatment modality for edentulous patients. Objective information regarding the number of necessary implants (i.e. two vs. four) for overdenture treatment is sparse. Treatment morbidity, expense and complexity are directly proportional to the number of dental implants used. Treatment decisions regarding this issue are difficult to make based upon available scientific data. The hypothesis of this investigation is that the four implant overdenture treatment is equivalent to the two independent implan treatment and can be measured in terms of prosthesis retention, stability, supporting and peri-implant tissue response, patient satisfaction/preference and treatment complications/failures. The goal of this prospective investigation is to scientifically compare the four implant overdenture to tha of the two in a clinical setting. A within-subject comparison using a four period, six sequence, three treatment crossover experimental design will allow all clinical subjects to receive all variable treatments. The specific aims of the study are: (1) investigate the possible equivalence of retention and stability; (2) identify changes, if any, which may occur in the supporting mucosal and peri-implant tissues; (3) identify patient satisfaction and preference; and (4) identify complications and failures for each treatment modality. Thirty healthy edentulous subjects will be used. Each will have four dental implants placed into the anterior mandible. New dentures will be fabricated to accommodate placement of implant/denture attachments. After successful implant/tissue integration, subjects will be randomly assigned to one of six sequences of treatments (A-B-C-C; A-C-B-B; B-A-C-C; B-C-A-A; C-A-B-B; C-B-A-A) where treatment A will utilize two bilateral implants which will be connected as independent units to the mandibular denture using o-ring attachments; treatment B will utilize two implants and a bar/clip denture attachment; and treatment C will utilize four implants and a bar/clip denture attachment. Each group will be followed for data collection for at least twelv months. The fourth period will allow estimation of the carry-over effect separate from the treatment effect. Quantitative measurement of retention and stability will be accomplished using force measurement instrumentation. Tissue health and treatment complications will be examined using a criterion based scoring system. Questionnaires will be used to assess subject satisfaction/preference.